Conventional elevator system suspensions can be characterized by the mechanical properties of the transmissive elements which connect three major elevator components: the car platform, the supporting frame, and the guide rails. The conventional elevator car platform is typically attached to the supporting frame with hard rubber pads. The frame, in turn, runs along the guide rails, which are supported either by stiffly sprung wheels or sliding gibs at four attachment points.
The motion of the car platform in these conventional elevator systems is affected by forces which act directly on the car, e.g., reactive forces due to passenger motion or wind forces, and by guide rail irregularities, e.g., butt joint misalignments, or waviness due to settling of the building. These conventional elevator suspension systems can be classified as "passive" in the sense that no energy is provided to the suspension system to counteract the direct or rail induced forces. For such passive systems, there is an inherent compromise in ride quality. Stiff transmissive elements mitigate the effects of direct car forces, while compliant (low stiffness) transmissive elements mitigate the effects of guide rail irregularities.
In U.S. Pat. No. 4,899,852 issued to Salmon et al, a passive suspension configuration is disclosed with a mechanically compliant attachment between the car platform and frame. The mechanically compliant attachment is realized by suspending the car platform from the frame with long steel rods. This elevator configuration, hereafter referred to as the "pendulum car" is a passive design in which the mitigation of the effects of rail irregularities is maximized at the expense of an increased sensitivity to direct car forces.
In a non-pendulum cab disclosure, U.S. Pat. No. 4,754,849, Hiroshi Ando shows electromagnets disposed outside the car symmetrically about guide rails in a control system using opposing forces from the electromagnets to keep the car steady using the rails as the necessary ferromagnetic mass but, rather than using the rails as a straight reference line, instead using a cable stretched between the top and bottom of the hoistway. The position of the car with respect to the cable is controlled using detectors in a closed loop control system. There is serious question as to whether such a cable can be successfully used as a reliable guide of straightness.
In another non-pendulum cab disclosure, U.S. Pat. No. 4,750,590, Matti Otala discloses what appears to be an essentially open loop control system with solenoid actuated guide shoes that uses the concept of memorizing the out-of-straightness of the guide rails for storage in a computer memory and then sensing the position of the car in the hoistway for the purpose of recalling the corresponding information from memory and correcting the guide rail shoe positions accordingly. An acceleration sensor is mentioned in claim 6 but does not appear to be otherwise disclosed as to its purpose in the specification or drawing. Perhaps it is used to determine the acceleration of the car in the hoistway. Such an acceleration signal would presumably be needed to determine which data point to retrieve from memory as suggested in claim 2. Otala's approach suffers from the problem of changes in the out-of-straightness before a correction run can be effected and the accuracy with which the stored information can be made to conform to the car's actual position.
A mounting arrangement for a pendulum-type or hung cab is shown in U.S. Pat. No. 4,113,064 by Shigeta et al wherein the cab is suspended within and from the top of an outer car framework by a plurality of rods connected to the bottom of the cab. A plurality of stabilizing stoppers are shown interposed between the underside of the hung cab and the floor of the car frame. Each stopper comprises a cylinder extending downward from the underside of the hung cab surrounding a rubber torus placed on an upright rod extending from the floor of the car frame. Clearance between the cylinder and the hung cab is sufficient to permit movement but insufficient to allow the hung cab to strike the car frame. Another embodiment comprising "bolster" means having ball bearings permits movement in any direction of the horizontal plane.
Another approach is disclosed by Luinstra et al in U.S. Pat. No. 4,660,682 wherein a pair of parallel rails are arranged horizontally in a parallelogram between the suspended cab and car frame with followers arranged to roll or slide on the rails in such a way that the hung cab can move in any horizontal direction relative to the car frame.
Both of the last two pendulum or supported cab approaches employ passive restraints on movement which by nature are reactive rather than active.
Active suspension systems are known in the automobile art. In particular, what we call "tunable shock absorbers" are used as tunable impedances. They comprise a relative displacement device made up, from a "systems" point of view, of a mechanical impedance (defined here as the frequency dependent ratio of deflection over applied force) of a stiffness in parallel with a damper. The stiffness and dampening elements are adjusted during different conditions. For example, during a cornering mode, as sensed by accelerometers, increased stiffness is desired on selected shock absorbers. Similarly, during braking, both front shocks are made stiffer. This is done in software by sensing the displacement of the car with respect to the frame and commanding a desired displacement. In simply adjusting stiffness and damping there is a trade-off; as the mechanical impedance of the shock absorbers is increased, the car becomes more sensitive to a bumpy road. Or, as the mechanical impedance of the shock absorbers is decreased, the car becomes more susceptible to direct forces, other than bumpiness.
In our study of improved ride quality, we compared the frequencies of disturbances caused by rail bumpiness in elevators to the frequency manifestations of direct forces and found, at least for elevators, a critical area of between two to ten Hertz where we could not satisfy both our desire to reduce mechanical impedance to cure rail bumpiness and our desire to increase mechanical impedance to mitigate direct forces. At least for elevators, this problem very significantly limits the effectiveness of the tunable impedance active suspension approach used in automobiles.